Ion implantation is a process of depositing chemical species into a substrate by direct bombardment of the substrate with energized ions. In semiconductor fabrication, ion implanters are used primarily for doping processes that alter the type and level of conductivity of target materials. A precise doping profile in an integrated circuit (IC) substrate is often crucial for proper IC performance. To achieve a desired doping profile, one or more ion species may be implanted in different doses, at different energies and different incident angles.
In production ion implanters, an ion beam is typically of a smaller size than a target wafer, which necessitates either scanning of the ion beam, scanning of the wafer, or a combination thereof. Scanning an ion beam typically refers to movement of the ion beam to increase wafer area that can be implanted, while scanning a wafer typically refers to the relative movement of a wafer through an ion beam. As used hereinafter, “scanning” refers to the relative movement of an ion beam with respect to a wafer or substrate surface. The ion beam is typically either a “ribbon beam” having a rectangular cross section or a “spot beam” having an approximately circular or elliptical cross section. For purpose of the present disclosure, a ribbon beam may be either a static ribbon beam or a scanned ribbon beam which is created by scanning a spot beam at a high frequency. In the case of a ribbon beam with a dimension larger than the wafer diameter, ion implantation of the wafer may be achieved by keeping the ribbon beam stationary and simultaneously moving the wafer across the ribbon beam in a direction orthogonal to the longer dimension of the ribbon beam. The one-dimensional (1-D) movement of the wafer may cause the ribbon beam to cover the entire wafer surface. In the case of a spot beam, scanning of a wafer may be achieved by sweeping the spot beam back and forth between two endpoints to form a scan path and by simultaneously moving the wafer across the scan path.
Sweeping of an ion beam may be accomplished through the use of electrostatic scanners or magnetic scanners, wherein the ion beam is controllably deflected from its normal trajectory to span a larger area by changing the electric or magnetic fields respectively in a direction orthogonal to the direction of travel of the ion beam. The strength of the scanner field determines the total deflection from the normal path of the ion beam, hence the ion beam may be scanned by changing the field strength of the scanner elements. The movement of the wafer across the scan path may be either continuous or incremental.
During ion implantation, it is desirable to achieve a uniform ion dose or beam current profile along the scan path. The process of tuning the ion implanter system to achieve the uniform ion dose or beam current profile is called “uniformity tuning.” Existing uniformity tuning techniques typically follow one of three approaches.
A first approach is to scan a spot beam across a wafer plane while moving the target wafer through the scan path at a constant velocity. In this first approach, improvement of dose uniformity is achieved by only tuning a beam scan velocity profile. Since the beam current distribution within the spot beam typically has a Gaussian-like non-uniform profile, it is often necessary to scan the spot beam off the wafer edges in order to avoid current fall-off at either end of the scan path. As a result, a significant fraction of the available beam current is lost due to overscan of the ion beam.
A second approach is to scan a target wafer through a stationary ion beam. If the ion beam cross section is smaller than the wafer diameter, the wafer is scanned in two directions with constant velocity in one direction and a step size in the other direction. This approach is known in the art as “2-D mechanical scan.” In this second approach, the wafer requires multiple passes in the first direction through the ion beam, and an optimized step size in the second (orthogonal) direction, which causes the ion implanter to operate at a low throughput. In addition, since the ion beam is typically non-uniform, the best possible uniformity achieved with the second approach is limited by the step size between the passes and the velocity in the first direction.
A third approach is to have a stationary ribbon beam that spans a distance larger than the wafer diameter, such that the wafer may be scanned across the ribbon beam to get a desired dose. The desired dose uniformity on the wafer is limited by the uniformity of the ion beam density distribution in the ribbon beam since the wafer is typically scanned at constant velocity. However, tuning the ribbon beam for a desired uniformity may be cumbersome and time consuming, thus may negatively impact the ion implanter's throughput.
In view of the foregoing, it would be desirable to provide a technique for improving ion implantation throughput and dose uniformity which overcomes the above-described inadequacies and shortcomings.